Section 6: Hot-Mix Asphalt Pavement Mixtures

6.1 General

Hot-mix asphalt (HMA) is a generic term that includes many
different types of mixtures of aggregate and asphalt cement (binder)
produced at elevated temperatures (generally between 300-350ºF) in
an asphalt plant. Typically, HMA mixtures are divided into three
mixture categories: dense-graded; open-graded; and gap-graded as
a function of the aggregate gradation used in the mix.

A variation on traditional hot-mix asphalt is warm-mix asphalt
(WMA). WMA technologies are processes or additives to HMA that allow
mixture production and placement to occur at temperatures (30-100ºF)
lower than conventional HMA without sacrificing performance. Technology currently
used to make the WMA process possible are chemical or organic binder
additives, chemical mixture additives, foaming admixtures, and binder
foaming using water-based plant modifications.

Additives and processes pre-qualified for use on department
projects can be found on the approved
WMA
list.

The following information addresses HMA mixtures, but generally
does not differ appreciably from WMA. Typically, the same parent
test procedures or specification item applies to both types of asphalt
mixtures.

Dense-graded mixes are produced with
well or continuously graded aggregate (gradation curve does not
have any abrupt slope change) and are specified under the current
Items 340 (Small Quantity) and 341. Typically, larger aggregates
“float” in a matrix of mastic composed of asphalt cement and screenings/fines
(see Figure 3-3).

Open-graded mixes are produced with
relatively uniform-sized aggregate typified by an absence of intermediate-sized
particles and low proportion of fines particles (gradation curve
has a nearly vertical drop in intermediate size range). Mixes typical
of this structure are the permeable friction course (Item 342) and
asphalt-treated permeable bases. Because of their open structure,
precautions are taken to minimize asphalt drain-down by using modified
binders (A-R) “asphalt rubber,” or by use of fibers. Stone-on-stone
contact with a heavy asphalt cement particle coating typifies these mixes
(see Figure 3-4).

Gap-graded mixes use an aggregate
gradation with particles ranging from coarse to fine with some intermediate
sizes missing or present in small amounts. The gradation curve may
have a “flat” region denoting the absence of a particle size or
a steep slope denoting small quantities of these intermediate aggregate
sizes (see Figure 3-5). These mixes are also typified by stone-on-stone
contact and can be more permeable than dense-graded mixes (Item
344, Superpave Mixtures), or highly impermeable (Item 346, Stone-Matrix
Asphalt).

Stone-matrix asphalt (SMA) will be missing most intermediate
sizes but have a relatively high proportion of fines. Fibers or
modified binders (A-R) are combined with these fines to build a
rich mastic coating around and between large aggregate particles.
Compacting and hand-working these mixes are usually more difficult
than with either dense-graded or open-graded mixes.

6.2 HMA Mix Design

Mix design is performed in a laboratory using one of the procedures
outlined in
Tex-204-F, “Design
of Bituminous Mixtures,” where the applicable procedure varies according
to mixture categories outlined above. In addition, material quality,
aggregate gradations, and other mixture requirements are given in
each of the specific mix standard or special specifications.

Resistance
to Permanent Deformation. The mix should not distort
or displace under traffic loading. The true test will come during
high summer temperatures under slow or standing truck traffic that
soften the binder and, as a result, the loads will be predominantly
carried by the aggregate structure.

Resistance
to Fatigue and Reflective Cracking. Fatigue and reflective
cracking resistance is inversely related to the stiffness of the
mix but proportional to asphalt film thickness. While stiffer mixes
are desirable for rut resistance, design for rut resistance alone
may be detrimental to the overall performance of the HMA mat if
fatiguing or reflective cracking occurs. Stiff mixtures perform
well when used in thick HMA pavements and can perform well when
used as a thin overlay on a continuously reinforced concrete pavement
(CRCP).

Thin HMA mats placed on an unbound base or on surfaces
prone to reflective cracking (e.g., jointed rigid pavements, bound
bases subject to shrinkage cracking, etc.) should use a mix that strikes
a better balance between rut and crack resistance. Fatigue and reflective
crack resistance is primarily controlled by the proper selection
of the asphalt binder. Application of a specialty designed crack-resistant
interlayer is another option for mitigating cracking.

Resistance
to Low Temperature (Thermal) Cracking. Cooler regions
of Texas are particularly confronted with thermal cracking concerns.
Thermal cracking is mitigated by the selection of an asphalt binder
with the proper low temperature properties.

Durability. The
mix must contain sufficient asphalt cement to ensure an adequate
film thickness around the aggregate particles. This helps to minimize
the hardening and aging of the asphalt binder during both production
and while in service. Sufficient asphalt binder content will also
help ensure adequate compaction in the field, keeping air voids
within a range that minimizes permeability and aging.

Resistance to Moisture Damage
(Stripping). Loss of adhesion between the aggregate
surface and the asphalt binder is often related to properties of
the aggregates. The assumption on the part of the mix designer should
be that moisture will eventually find its way into the pavement structure;
therefore, mixtures used at any level within the pavement structure
should be designed to resist stripping by using anti-stripping agents.

Workability. Mixes
that can be adequately compacted under laboratory conditions may
not be easily compacted in the field. Adjustments may need to be
made to the mix design to ensure the mix can be properly placed
in the field without sacrificing performance.

Skid Resistance. This
is a concern for surface mixtures that must have sufficient resistance
to skidding, particularly under wet weather conditions. Aggregate
properties such as texture, shape, size, and resistance to polish
are all factors related to skid resistance. Under the department’s
Wet
Surface Crash Reduction Program Guidelines (WSCRP)2, aggregates are classified into
three categories (A, B, or C) based on a combination of frictional
and durability properties. A friction demand assessment is made
by the engineer. The proper aggregate or blend (using categories
A and B only) to achieve the assessed rating is then selected.

Design is facilitated by the use of a series of
automated
mix design programs in Excel format. Mix designs can be generated
in accordance with Tex-204-F by either department personnel or by
a consultant/contractor who is certified by the department-approved
hot-mix asphalt certification program. Plant mix or raw materials
must be furnished by the contractor to the department project engineer
to allow verification of the mix design.

6.2.2 Texas Gyratory Compactor (TGC)

For dense-graded hot-mix asphalt (Types A, B, C, D, and F
of Items 340 and 341), the Texas gyratory compactor (TGC) is used
to compact sample mixtures in accordance with
Tex-206-F, “Compacting
Specimens Using the Texas Gyratory Compactor [TGC].” Item 347, Thin
Overlay Mixture [TOM], often associated with pavement preservation
operations, can also be compacted using the TGC. The TGC uses a
4.0-in. diameter mold, with a target specimen height of 2.0 in.

Compactive effort is achieved by a combination of gyratory
compactions governed by achieving a low pressure threshold, followed
by uniform axial compaction achieving a high pressure threshold. Optimum
asphalt binder content is derived by molding specimens at various
binder contents, plotting the asphalt vs. density curve and selecting
the binder content that corresponds to the specified target laboratory
molded density.

The SGC uses a 6.0-in. diameter mold with a target specimen
height of 4.5 in. The larger diameter mold allows retention of material
3/4-in. or larger in the compacted sample whereas Parts I and II of
Tex-204-F require removal of this material because of the smaller
TGC mold size. Samples are prepared at various asphalt binder contents
around the estimated optimum content. Plots are generated within
the design software to evaluate optimum binder content at the specified
target laboratory molded density and to ensure other key parameters
are met.

Mixture designs using the SGC are also controlled by the number
of gyrations (N) required to achieve proper density. Depending upon
the mix type, an N design (Ndes) related to minimum asphalt content
and design air voids is established in each mixture specification.
Ndes can be adjusted to ensure sufficient asphalt cement content
and mix workability.

Traditionally, lab-molded specimens have been produced at
an AC content that will yield a target density of 96% of the theoretical
maximum density. Some variation is allowed to ensure mixes are workable
under field compaction conditions, thus mitigating tendencies toward
very dry mixes and improving field achieved densities.

6.2.4 Voids in the Mineral Aggregate

Another mix design parameter that has significant impact on
mix workability and durability is the voids in the mineral aggregate
(VMA). Conceptually, this is the volume of space within a mix that is
available for asphalt binder to occupy; as a result, this mixture
design parameter has a direct impact on the binder film thickness.
For this reason, minimum values are placed on this parameter, specific
to the mix type and gradation. Related to VMA is the voids filled
with asphalt cement (VFA), the percent of the volume of VMA that
is filled with asphalt cement. A range of acceptable VFA is a further
control placed on Superpave mixtures.

6.2.5 Evaluating Mix Stability

Historically, mix stability for the traditional dense-graded
mixes was evaluated using the Hveem stabilometer. The lab-compacted
samples were subjected to axial compression and shearing resistance
of the mixture was evaluated.

A more comprehensive evaluation of all hot-mix asphalt mixtures
for problems related to stability and moisture susceptibility (with
the exception of permeable friction course and mixtures using asphalt-rubber
modified binders) is now accomplished using the Hamburg Wheel Tracking
Device, or simply Hamburg (see
Tex-242-F,
“Hamburg Wheel-tracking Test").

For the case of a dense-graded mixture designed using the
Texas Gyratory Compactor (TGC), once optimum asphalt cement is determined,
new samples are molded to 93% theoretical maximum density in the
Superpave Gyratory Compactor (SGC) using the optimum asphalt content
(a requirement for all lab-prepared Hamburg test specimens).

The Hamburg test uses a pair of abutting, trimmed SGC samples
placed in a 122°F (50ºC) water bath. A weighted steel wheel passes
back and forth across the surface; rut depth is evaluated per number
of passes. A minimum threshold of passes resulting in a rut depth
no greater than 1/2-in. is established based on the PG binder grade.

6.2.6 Tools to Improve HMA Mixes

Research project 0-5123 developed a methodology to design
a balanced HMA mixture, considering both rutting (Hamburg) and fatigue
(Overlay Tester) properties.

The Overlay Tester has been implemented for select mixtures
using test method
Tex-248-F.

The result of the mix design process is a job-mix formula
(JMF), a starting point for the contractor in producing HMA for
the project. The engineer and contractor generally verify the JMF
based on plant-produced mixture from a trial batch. The engineer
may accept an existing mixture design previously used by the department
and may waive the trial batch to verify the JMF. It is recommended that
if the trial batch is waived, the mix design should have been developed
and verified within the past 12 months.

If the JMF fails the verification check using the trial batch,
the JMF is adjusted or the mix may be redesigned. Additional plant-produced
trial batches are run until the JMF is verified. During the course
of the project, the JMF may be modified without developing a new
mix design to achieve specified requirements as long as adjustments
do not exceed tolerances established within the applicable mix specification.

6.3 Guidelines for Selecting HMA Mixtures

Selection of an HMA mix or combination of mixes to use in
a project should be a conscious decision made by the engineer based
on mix attributes; evaluating suitability as part of the overall pavement
design, existing pavement conditions, lift thickness, traffic loading
characteristics, environment, past performance, local contractor
experience, and economics.

Guidelines have been established to assist in the decision-making
process in the form of a
Mixture Selection
Guide. The Guide provides general descriptions of the various
HMA mixes used in the state (typical use, advantages, disadvantages);
table ratings (subjective) of mixture characteristics for each of
the mixture types; table of typical lift thicknesses; location within
a pavement structure for each mixture type; and recommended choices
for surface mixtures.

The department is required to establish and maintain a program
to ensure that pavements with good skid resistant characteristics
are used. This program is commonly referred to as the
WSCRP3.

The department is charged with developing and implementing
methodologies for the detection and improvement of locations with
a significant incident of wet surface accidents using accident record systems
and countermeasures to address those locations.

The department is also directed to utilize methods for the
analysis of the skid resistant characteristics of selected roadway
sections to:

provide information for use in developing
safety improvement projects and the implementation of cost effective
treatments at appropriate locations.

The WSCRP allows the department to take advantage of the increased
knowledge gained through our research efforts and to more effectively
and efficiently address the various regional demands of Texas pavements.
WSCRP addresses three separate but interrelated phases of pavement
friction safety. The three phases are crash analysis, aggregate
selection, and skid testing.

Crash analysis
is the first phase and it consists of the identification, evaluation,
and improvement (as needed) for all wet surface crash locations.
The Traffic Operations Division publishes annual crash reports at
the following location:
http://crossroads/org/trf/4. Under Quick Links, click
on “District Wet Surface Crash Reduction Program Location Reports”
to see annual crash reports by district.

The second part of the program is aggregate
selection. Each bituminous coarse aggregate source is classified
into categories based on a combination of the frictional and durability
properties of the aggregate. The classifications will be listed
in the
Bituminous
Rated Source Quality Catalog (BRSQC) updated (every 6 mos.)
by the Geotechnical, Soils & Aggregates Branch of the Construction
Division.

The third part of the program will consist
of skid analysis and will include a mandatory collection of skid
data that will become part of the new Pavement Management Information
System (PA).

Although the Geotechnical, Soils & Aggregates
Branch of the Construction Division has been delegated
responsibility for administering
WSCRP5, it is the district’s responsibility
to manage frictional properties for their pavements through sound
engineering judgment and application of the program.